Generally, virtual environments may be divided into two broad categories, virtual reality and artificial reality, each of which may be enhanced with a system of augmented reality.
Virtual reality is a known process of actively stepping inside (to see, hear, act upon) a computer generated, virtual environment. It usually assumes the use of a head-mounted audio/video display, and position and orientation sensors, such as are described in A. Wexelblat (editor), Virtual reality applications and explorations, Academic Press, 1993; and B. MacIntyre and S. Feiner, Future of multimedia user interfaces, Multimedia Systems, (4): 250–268, 1996.
Artificial reality is a known process of describing virtual environments such that the user's body and actions combine with the computer generated sensory information to forge a single presence. The human perceives his actions in terms of the body's relationship to the simulated world, such as is described in M. Hein, The metaphysics of virtual reality, Oxford University Press, 1993, and M. W. Krueger, Artificial Reality II, Addison-Wesly Publishing Co., Reading, Mass., 1991.
Augmented reality is a known technology where the user's display shows a superposition of the real world and computer generated graphics (to augment the presentation of the real world objects) by means of a see-through display, such as is described in T. P. Caudell, Introduction to Augmented Reality, SPIE Proceedings, vol. 2351: Telemanipulator and Telepresence Technologies, pp. 271–281, Boston, Mass., 1994.
There are a number of known spatial tracking solutions presently used in virtual reality systems, such as are described in MacIntyre et al., supra, and in R. Allison, et al., First steps with a ridable computer., Proceedings of the Virtual Reality 2000 conference, IEEE Computer Society, 18–22 May 2000, pp. 169–175. Mechanical, electromagnetic, ultrasonic, acoustic, and optic (vision-based) systems are known. It is also known to exploit non-visual cues of motion from devices that can be physically moved to generate such cues, such as is described in Caudell, supra. Six-degree-of-freedom sensors are known to provide both position and orientation information in 3-D. Mechanical tracking systems are known that rely on a motion-tracking support structure of high precision, e.g., using opto-mechanical shaft encoders (BOOM 3C from Fakespace Labs). The user is generally anchored to the mechanical device. Electromagnetic systems (e.g., Flock products from Ascension Technology) use DC magnetic fields generated by three mutually orthogonal coils from a stationary transmitter that are detected by a similar three-coils receiver. The audio tracking system produced by Logitech uses three fixed ultrasonic speakers and three mobile microphones, thus detecting all possible 9 distances. Computer vision-based systems use either fixed cameras that track objects with markings (e.g., Northern Digital's Polaris product), or mobile cameras attached to objects that watch how the world moves around (see MacIntyre, supra). Global Positioning System (GPS) based systems receive signals from positioning satellites either directly, or in conjunction with an additional ground-located receiver and transmitter in a precisely known position. Small sized receivers with a small price also make their way into mobile devices (e.g., The Pocket CoPilot from TravRoute).
Many virtual environment applications try to mimic the real world. Thus it would be ideal if user interaction replicated the user's natural way of interacting with the real objects. Almost all VR applications involve some kind of navigation through a virtual 3D environment. Navigation in such environments is a difficult problem: users often get disoriented or lost. A number of three degrees of freedom input devices, including 3D mice, spaceballs, and joysticks have been designed to facilitate user interaction. However, three degrees of freedom are often not sufficient to define user position and orientation in a 3D scenario.
What is needed is a way to localize and receive commands from a user in a virtual environment system without need for the user to have special localizing equipment attached to him nor to input commands into a manual input device, such as a keyboard or mouse.